1. Technical Field
The present invention relates to a microelectronic pressure sensor allowing the capture of the profile of a surface when said surface is applied on the sensor. The invention is particularly useful for the fingerprint recognition.
2. Description of the Related Art
Classically, a fingerprint sensor operates on the basis of a capacitive measurement. The finger is applied on a substantially planar surface which comprises a high number of first electrodes of different capacitances. The finger, being electrically conductive, embodies a second electrode which is common to all those capacitances. The planar surface is covered by an electrical isolator. The distance separating the second electrodes (i.e., the finger) and the first electrodes is not constant and varies according to the rugosity of the finger. The values of the capacitances being measured are then representative of a fingerprint profile, and thus characterizes the fingerprint.
In order to reduce the number of the capacitors, it has been suggested to scan the asperity of the finger by means of a series of capacitive elements. Such a device is described in U.S. Pat. No. 6,317,508.
The measurement of one capacitor shows to be a delicate operation to achieve, requiring a complex measurement device. Furthermore, the accuracy of such measurement closely depends on the electrostatic environment of the sensor since the electrical circuits therewith associated have to measure very low currents flowing through high impedance circuits. The conductivity of the finger or the pollution on the surface of the sensor might spoil the measurement of the capacity being associated with the profile of the finger.
In the background art, there is already known the use of MOSFET transistors fitted with a mobile gate. Patent application WO 03/078299 and U.S. Pat. No. 7,304,358 describe two distinctive embodiments of such a transistor. As known by a skilled man, a MOSFET transistor is created by means of a known technique. There is then substituted during the manufacturing process of the MOS transistor, at least locally, one sacrificial layer to the gate oxide existing between the substrate and the mobile gate of the MOS Transistor. At the end of the process, the withdrawal of the sacrificial layer, with that of the possible layers already laid on the gate causes the creation of a mobile gate with respect to the substrate located below.
The skilled man is aware of diverse processes for realizing such a transistor and the various possibilities for carrying out a mobile gate. Patent application WO 03/078299 and U.S. Pat. No. 7,304,358 disclose an overhanging gate and a gate which can be distorted (by bending), which are two particular examples of a mobile gate. Those manufacturing processes can be made compatible with the standard MOS manufacturing techniques used for the creation of MOS circuits. Mobile gate transistors can be realized in an integrated circuit which further includes functions which are embodied by MOS transistors having each a standard non-mobile gate. However, the mobile gates show to be very fragile and small elements and are never set in a mechanical interaction with elements being exterior of the integrated circuit. Such interaction would irreversibly destroy the gate or would result in the chemical pollution of the MOSFET transistors, eventually spoiling them.